554 PROF. E. R. LANKESTER ON THE HEART [June 20, 



ing to the septal part of the valve, are smaller in size, and in passing 

 from left to right dwindle, so that at the right of the middle line the 

 chordae are inserted directly into the ventricular wall and not into 

 papillary eminences. Only three distinct papillary eminences can 

 be distinguished in this series. 



Comparing this arrangement with that found in Man, it is obvious 

 that the seven papillary muscles of the anterior part of the valve cor- 

 respond to the "great" and the "right" anterior papillary muscles of 

 Man ; whilst the superior series connected with the septal part of the 

 valve correspond to the posterior papillary muscles of Man. But in the 

 Rabbit not only must we consider that the " great " and the " right " 

 papillary muscles are divided and represented each by three separate 

 papillary muscles, but also that the attachment of chordae springing 

 from the extreme left of the left anterior cusp and from the left of 

 the septal cusp in the human heart, are in the Rabbit attached to 

 small papillary elevations of muscular substance, instead of being 

 attached simply to the unraised surface of the ventricular wall. 



It is an interesting question as to whether the condition found in 

 Man and in most mammals is more primitive tlian that found in the 

 Rabbit. The fact that the Rodents are lower forms than the 

 Simise might lead us to regard the condition seen in the Rabbit as 

 more primitive ; but the very general conformity of the other Mam- 

 malia (including such Marsupials and Edentata as have been examined) 

 to the arrangement found in Man, leads to the supposition that the 

 Rabbit's right cardiac valve is a specialization departing from the 

 earlier type preserved in Man. 



This conclusion will be found to be confirmed by the facts which 

 we now shall expose in reference to the right cardiac valve of Ornitho- 

 rhynchus. 



C. Of Ornithorhynchus paradoxus. — The right auriculo- 

 ventricular valve of Ornithorhynchus is drawn in two sets of figures, 

 accompanying this paper (Plates XXXIX., XL.), taken from the 

 two female specimens in an excellent state of preservation mentioned 

 above. In both of them the membranous collar which forms the 

 complete valve in other Mammalia is seen to be incompletely de- 

 veloped, and not "entire" as stated by Gegenbaur. Instead of the 

 elliptical auriculo-ventricular orifice bein-^ completely fringed by the 

 more or less deeply dependent valve, we find only its anterior margin 

 and a small portion of its posterior or septal margin (the extreme 

 right) thus fringed. The septal or posterior portion of the valve 

 is, in fact, almost entirely wanting in one of my specimens (tig. 16). 

 This result of my observations is diametrically in opposition to the 

 statements of Gegenbaur quoted at the commencement of this memoir. 



The absence of the greater part, or even the whole, of the septal 

 flap at once constitutes a very important difference between the right 

 cardiac valve of Ornithorhynchus and that of any other Mammal 

 which is known. 



Anterior flap. — The well-developed anterior portion of the membra- 

 nous valve is triangular in form, and is connected with a very large mus- 

 cular column, which for convenience may be spoken of as musculus 



